Why does my power bank deliver less than its rating?

Cell voltage is 3.7 V and USB output is 5 V, so conversion loses about twenty percent before anything else. A 10,000 mAh bank delivers around 6,000 to 7,000 mAh at 5 V.

What does a 500 cycle rating mean?

That after 500 full cycles the battery holds about eighty percent of its original capacity. It does not stop working, it just holds less.

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Battery Life Calculator

Battery runtime and cycle life. Nameplate capacity is never fully available: fifteen percent or so is lost to cutoff voltage, ageing and the Peukert effect at higher discharge rates.

Also called: battery runtime calculator, mah to hours.

Runtime

17 hours 0 minutes

17 hours 0 minutes from 5,000 mAh at 250 mA, using 85% of nameplate capacity. That is 18.5 Wh of energy. At 1 cycles a day the 500 rated cycles last about 1.37 years. Nameplate capacity is never fully available: cutoff voltage, ageing and high discharge rates all take a share.

Runtime in hours

Energy stored

18.5

Usable capacity

4,250

Discharge C rate

0.05

Calendar life

1.37

On real capacity

Nameplate capacity is never fully available: cutoff voltage, ageing and high discharge rates all take a share.

How this is calculated

Method and background

How this is calculated

Runtime is usable capacity divided by current draw. Usable is below nameplate for three reasons: the device cuts off before the cell is empty, capacity fades with age, and drawing current faster than about the rated C rate delivers less total energy, which is the Peukert effect. The C rate shown is the discharge relative to capacity, and a rate above 1C is where the derating becomes noticeable. Cycle life is quoted to eighty percent of original capacity rather than to failure.

usable capacity over current, which understates for high discharge rates because of the Peukert effect

C: Capacity
u: Usable share

Worked examples

Each of these is asserted on every build. If a change to the engine ever moved one of these answers, the build would fail before the page could print it.

a 5000 mAh cell at 250 mA

Battery capacity: 5,000 mAh
Battery voltage: 3.7 V
Average current draw: 250 mA
Usable share of capacity: 85%
Rated cycles: 500
Cycles a day: 1

Runtime17 hours 0 minutes

4250/250; 5 Ah x 3.7 V

Open this example

full usable capacity gives the nameplate runtime

Battery capacity: 5,000 mAh
Battery voltage: 3.7 V
Average current draw: 250 mA
Usable share of capacity: 100%
Rated cycles: 500
Cycles a day: 1

Runtime20 hours 0 minutes

boundary

Open this example

Method and limits

What it assumes

A constant current draw, which most real devices do not have.

What it deliberately does not model

Cold reduces available capacity substantially, often by twenty percent or more.
The Peukert effect is not modelled explicitly, only allowed for in the usable share.
Rated cycles assume a specified depth of discharge, usually shallower than full.

Formula version 1.0.0 · definition 1.0.0 · India · Report a problem with this calculator

Frequently asked questions

Why does my power bank deliver less than its rating?: Cell voltage is 3.7 V and USB output is 5 V, so conversion loses about twenty percent before anything else. A 10,000 mAh bank delivers around 6,000 to 7,000 mAh at 5 V.
What does a 500 cycle rating mean?: That after 500 full cycles the battery holds about eighty percent of its original capacity. It does not stop working, it just holds less.